Hmmm.....ok. Tell me about the truck (tranny, rear gears, and 4x2 or 4x4) and about the trailer being used (size, bumper hitch, goose neck, etc). I pull trailers alot. Cattle and horses. I've used different sized trucks, too, from 302 F150 to V10 Superduty. If it's just like a two horse bumper hitch (receiver hitch) type trailer with a couple horses then the F150 302 will do fine with either transmission. If he doesn't tow ALOT then the 3.55s will be fine. If he tows quite a bit then I'd go to 4.10s. 3.73s are kinda in the middle ya know what I mean. If the motor needs rebuilt then you could always over haul the heads and also port and polish them up. Also I'd try to get ahold of a GT40 intake. That'll make good hp and tq. If you want different heads then get a used set of GT40 cast irons and work them over before you put them on. Even if you didn't the GT40 heads would definitely help in the tq range. You might try a cam out of a 87-95 5.0 mustang if your 302 is roller. Those are a bit more aggressive than the stock cam in the truck and will still work fine with the speeddensity truck. They even work in our Lightning 351Ws with our speeddensity if we convert to roller (we just found that out from a guy on the board who is currently experimenting with his truck and will find out power gains/losses soon). A custom cam would still be the way to go over the stock cams.
Though his best bet would be to get a 351W
. We have a 93 F150 ext. cab 4x2 short bed with the 302 and E4OD tranny. We pull our two horse trailer with it and it does just fine.